Killua's activites

i read this one a long time ago and thought i had it bookmarked but i guess not.

the plot is about a father who is a professor, and his adopted son who is also interning at the same school/college. but the father thinks that the adopted son is actually the one who murdered his biological son.

the adopted son is in love with the father and the father finds him masturbating on his blanket and throws it away. i also remember the son having a casual friends with benefits relationship with one of his classmates who likes the adopted son.

please help me find this one. i can faintly recall the title as living with my son's murderer or something.